TORRESIAN IMPERIAL-PIGEON NESTING ERUPTION IN CAIRNS CITY
Contact Call | Volume 10 Number 2 | June 2021
HANZAB records that our Pied Imperial-Pigeons (PIP) nest mainly on islands along the tropical coast of Queensland with only a small number on the mainland. From 2012 on, however, I recorded an abrupt localised change to this pattern of behaviour.
There have always been a few Imperial-Pigeons nesting in Cairns. In the early 1950s, a pair nested each summer in a tall coconut palm in the Cairns Botanic Gardens near to our family home. They did not return after the palm was blown down in 1956 by Cyclone Agnes. There have also been suggestions from some older Cairns residents that there were in times past nesting eruptions in the mangroves at the northern end of the Cairns Esplanade but I have no knowledge of this and with no concrete records it has so far been impossible to verify.
During the summer of 2011–12 my brother Phillip alerted me to unusual PIP nesting activity in Cairns and I decided then to keep nest location records on this changed nesting behaviour. The data I planned to record were very detailed, and included number, geographical distribution, species of tree used, evidence of predators, success or failure of breeding and persistence of the phenomenon. In making these recordings a GPS track was to be kept in order to quantify effort. One thing I was not able to do, for reasons of logistics, was identify from the droppings, the suite of botanical species the birds use as food. This would have been a useful observation.
Torresian Imperial-Pigeon with a young in the nest in Cairns. Photo: Brian Venables.
The first three years we (there were others involved then) recorded less than 470 nests each season, mostly in Cairns city, but some were recorded in the peri-urban areas. (See reference (1) below). The nest records increased over the following years and this season (2020/21) my records include 1279 nest usages (as at 2021-02-21) all of which are in Cairns city and environs. My informal observations suggest that predators have increased also, although a link to the Imperial-Pigeon nesting remains to be tested. What is clear even to the casual observer is that Grey and Brown Goshawks have gone from rare to common in Cairns and their nests are now regularly reported. Rufous Owls were also virtually unknown in Cairns, but today there are at least two pairs nesting in suburban Cairns. Both the owl and the goshawks have been recorded preying on the Imperial-Pigeon chicks and adults. Black Butcherbirds have also been recorded preying on both nestlings and eggs.
One interesting feature of the distribution has been that the species seems to prefer isolated copses of trees within the city limits, with a distinct concentration along the foreshore. A noteworthy record is the use by the species of man-made structures on which to build nests. TV antennae, streetlights and installations on electricity poles have been used occasionally.
The main drivers of this sudden change in nesting behaviour remain unknown, but cyclone Yasi in February 2011 is probably implicated. TC Yasi severely damaged vegetation in the Hinchinbrook area including North Brook Island, traditionally used by a very large PIP aggregation, as well as the nearby mainland.
Torresian Imperial-Pigeons on the nest in Cairns. Photo: Brian Venables.
When the North Brook Island population returned after their 2011 winter migration, they would have found severely degraded habitat for nesting and foraging. Many birds probably dispersed temporarily to other areas. It's likely that some of the dispersing North Brook PIPs caused the sudden increase in Cairns.
It is now becoming increasingly clear, at least informally, that chicks hatched in Cairns city may have acquired an instinct to home back to where they were hatched. Presumably, this will continue until once again some environmental catastrophe in Cairns itself drives them elsewhere.
It is fascinating to me to hypothesise why the species nested on the islands in the first place. The islands offer safety from some predators but not much food. As they are frugivores they needed to commute each day from nest to the mainland to feed and back to the islands to tend the nest and roost at night. This strategy appears to offer safety but at a great energy cost. Could the expanse of water have given them a degree of predator protection then, that now the urban sprawl of Cairns provides, at a much less energy cost?
Long term monitoring of PIPs was started by Arthur and Margaret Thorsborne when they strenuously campaigned to save the North Brook Island population from extinction in the early 1960s. In the post WWII period, the species was at a low ebb, with much illegal hunting. The Queensland Government of the time showed no interest in enforcing their own protection regulations and Arthur and Margaret’s actions forced the Government’s hand to do so. As a result, PIP flight counts at North Brook Island have continued annually for more than 50 years, and for shorter periods at other islands.
More recently PIPwatch.net was set up by Dr Julia Hazel to collect mainland PIP observations and share data with Birdlife Australia. All my Cairns nest records go to the database of PIPwatch.net and my data for 2012-2015 were analysed in detail for an article published in the scientific journal Wildlife Research.
It is a pleasure to acknowledge the contribution of Dr Julia Hazel to this article.
(1) Hazel, Julia and Brian L. Venables. (2017) Can island specialists succeed as urban pioneers? Pied imperial-pigeons provide a case study. Wildlife Research 44: 40–47.
If you loved this article, why not subscribe to our newsletter Contact Call?